Virtual private networking (VPN) using TCP/IP enables secure, high speed communications between remote computing sites, using the internet as a communications medium. Information passing between sites across the internet may be protected against interception by unwanted eavesdroppers or malicious hackers by a variety of security measures. Effective security measures must, at a minimum, incorporate functions that ensure any or all of the following protections: Data integrity against the inadvertent or malicious modification of data during transit; prevention against denial-of-service attacks by employing anti-repeat measures; source authentication; confidentiality of source address and other header information during transit; and packet payload protection against unwanted interception. One standard model for providing internet security is the Internet Protocol Security suite, IPSec. IPSec works with the TCP/IP communications protocol to provide secure communications between devices connected to the internet or connected to private LANs (Local Area Networks) that are, themselves, connected to the internet.
The TCP/IP (Transmission Control Protocol/Internet Protocol) protocol suite uses IP addresses to identify each device on a network. A global IP address uniquely identifies a device on the internet Such devices may be computers, printers, routers, switches, gateways, or other network devices. Devices having global IP addresses may be directly referenced as a source or destination on the internet. The TCP/IP communications protocol, however, is not exclusively limited to the internet, but may be used on private LANs as well. Private LANs using TCP/IP frequently use “local” IP addresses for network devices. Although no two devices on a private LAN may share the same local IP address, private LANs are isolated from the internet, and local devices on the LAN are not visible from the internet. IP addresses for local devices therefore need not be “globally” unique. A LAN using local IP addresses will be connected to the internet through a gateway, which is a device that may filter or route messages between the LAN and the internet. Since the gateway is directly attached to the internet, and is visible to it, the gateway must have a globally unique IP address for communicating across the internet. However, since the LAN is not directly visible from the internet, local machines on the LAN do not require IP addresses that are globally unique.
TCP/IP is the communications protocol that is used on the internet. Information to be communicated using TCP/IP is contained in “datagrams.” A datagram consists of a discrete “packet” of information to which one or more headers are appended. Headers contain information needed by TCP/IP to direct the packet to its intended destination and to ensure its proper handling during transit. Each datagram is individually addressable, and may be a connection-oriented TCP datagram or a “connectionless” UDP (User Datagram Protocol) datagram. Each UDP datagram includes an IP header and a UDP header. The IP header contains at least a “source” IP address and a “destination” IP address, while the UDP header includes source and destination service addresses (port addresses, given as numbers). In IPv4, IP addresses are 32 bits in length, and are associated with the now-familiar xxx.xxx.xxx.xxx format. In this format, each three-digit segment is a binary octet representing a number between 0 and 255. A complete IP address combines the address of a logical network or network segment with the address of a “node” (device) on the network. The address of the network or network segment may encompass the first 3, 6, or 9 digits of the IP address. A device on the network or network segment is identified by a node address that consists of those remaining digits that are not used in the network or network segment address.
The source and destination service addresses contained in a UDP header are 16-bit numbers, variously known as “ports” or “sockets,” which are used to direct the packet to an intended process that is active on the sending or receiving device. The term “port,” or “port address,” as used herein, refers to a service address field in a UDP header. Although in theory there are as many ports as there are addresses in a 16-bit number, by convention many port addresses have been reserved for established processes. Thus, for example, port 80 is reserved for HTTP, and ports 20 and 21 are reserved for FTP. Through the use of port addresses, data arriving at a local machine running more than one process will be directed to the process for which it was intended. Where a process running on a local host is not one of the reserved processes, the local host may select any port number from a pool of unreserved port numbers to identify the “source” process. A reply packet referencing that port number in the “destination” field will be directed to the process.
With the explosive growth of internet usage during the last decade, and its projected growth in the future, globally unique IP addresses have become a scarce resource. In addition, many businesses maintaining private LANs have little or no need for each computer and device on the LAN to have a unique global IP address. Many such businesses would, in any event, prefer to maintain the confidentiality of their computers' IP addresses. Rather than waste limited global resources by giving each local device a unique global IP address, many private LANs utilize local IP addresses for devices on the LAN. In order to provide connectivity to the internet, such LANs will employ a single globally unique address to be used on the internet by the gateway separating the LAN from the internet.
Through the use of Network Address Translation (NAT) techniques, a gateway device separating a LAN from the internet can provide security as a firewall while enabling machines with local IP addresses to access the internet through the unique global address of the gateway. A device on a LAN may have a static local IP address, or it may have a local IP address dynamically assigned to it at log on. The gateway maintains a translation table with the local IP addresses for each device on the LAN. A UDP packet sent from a local machine and destined for the internet will have its local IP address and port address identified in the source fields of the IP and UDP headers, respectively. The gateway will receive the packet from the local machine, will substitute its external globally-unique IP address and a new port address (taken from a pool of unused, unreserved port addresses) into the source fields of the IP and UDP headers. It will next update the CRC (Cyclical Redundancy Check) and make any other necessary changes to ensure data integrity, then will send the packet to the internet. As part of the process, the gateway will update its internal translation table to cross reference the local machine's IP address with the source port address originally reported by that machine, with the new source port address assigned to the internet-bound packet, and with the destination IP address. Upon receipt of a reply from the internet, the gateway will recognize its own IP address in the packet header, and will examine the incoming packet's destination port address. If it finds the destination port address in its internal table, the gateway will substitute the cross referenced local machine's IP address and original port address into the destination fields of the packet, will update the CRC and any other necessary parameters, and then will dispatch the packet to the LAN, where it will be received by the local machine and directed to the appropriate process. In this manner, a number of computers on a LAN having only local IP addresses can communicate across the internet through a single globally unique IP address.
Although NAT gateways provide firewall security against direct accessing of the LAN from the internet, they do not provide security against interception or modification of a packet intended for the LAN while in transit on the internet, and they do not ensure “trustworthiness” from challenges originating within the LAN. Thus, the security provided by IPSec is a necessary protection for LANs that must maintain security while interfacing with the internet.
A common implementation of IPSec is to provide security for VPNs consisting of at least one main computing site and one or more remote LANs. The main site and remote LANs are connected across the internet, using that high speed medium to communicate between sites instead of significantly more expensive private leased lines. The drawback to using the internet as a transmission medium, however, is that the internet is inherently insecure, and provides little or no inherent protection against the snooping, detection, “spoofing,” or ultimate theft, modification or diversion of messages by hackers. Thus, there is a need for comprehensive security measures where secure data transmissions are required. The IPSec protocol implements security measures to ensure authentication of data and data integrity.
The IPSec protocol suite implements security at the network layer of the multi-layered OSI (Open Systems Interconnection) network reference model. The suite includes a number of separate protocols that are used in conjunction with one another to ensure the security of UDP datagrams that carry information across the internet. The base architecture of IPSec compliant systems is explained in RFC2401, “Security Architecture for the Internet Protocol,” S. Kent and R. Atkinson (November 1998). The AH (Authentication Header) protocol assures data integrity, source authentication, and incorporates “anti-repeat” measures to deter denial-of-service attacks. ESP (Encapsulation Security Payload) protocol provides protections similar to AH, but adds the additional feature of payload encryption. Both AH and ESP headers have a field for a Security Parameters Index (SPI). The SPI is a 32-bit pseudo-random value that is used to identify a Security Association (SA) for the datagram. Further information regarding these protocols may be found in RFC1826, “IP Authentication Header,” by R. Atkinson (August 1995), and RFC2406, “IP Encapsulating Security Payload (ESP),” S. Kent and R. Atkinson (November 1998). ISAKMP/Oakley (Internet Security Association and Key Management Protocol, also commonly referred to as Internet Key Exchange—IKE) is a handshaking protocol that establishes the parameters for a secure session between two hosts and provides for the exchange of keying and other security information that is used to implement the secure session and permit the transmission of encrypted data. The ISAKMP/Oakley protocol (hereafter referred to simply as ISAKMP) involves the initial exchanges of unencrypted messages to provide both machines with initialization data from which authentication may be established and secure keys for data encryption may be generated. An explanation of these processes may be found in RFC2409, “The Internet Key Exchange,” D. Harkins and D. Carrel (November, 1998). Once security parameters sufficient to establish Security Associations (SAs) between hosts have been exchanged, all subsequent transmissions will be encrypted and fully authenticated in accordance with the agreed-upon protocols. At that point the ISAKMP protocol terminates. Subsequent addressing is based upon the IP address for each machine and the machine's SPI for that session. The SPI is unique for each machine during a session. The gateway for a private LAN will maintain an internal table in which “SPI-in” in a value that is cross referenced to the local machine's IP address, and “SPI-out” is cross referenced to the IP address of the machine on the internet that is communicating with the local machine. The SPI for each machine is computed from information exchanged during the ISAKMP transmissions, and is carried in the AH or ESP header that is appended to UDP packets. Because IPSec protocols may be nested to provide security in a variety of environments, a single datagram may include both an AH and an ESP header, and may encrypt some header information.
Each of the foregoing security protocols modifies the UDP packet by placing new header information on the packet, modifying certain fields within the packet to conform to the protocol being used and, in some cases, encrypting the payload and all or parts of other packet headers. Thus, under IPSec, when a UDP datagram leaves a “secure” domain for transit across an untrusted network, it will normally consist of an IP header, an AH or ESP header (or both), and an encapsulated payload. Header information will include a destination address, an SPI, and sufficient SA information to ensure that the datagram reaches its destination and can be authenticated to the destination host. Encapsulation of the payload ensures that information contained within the payload is denied to unwanted eavesdroppers and hackers. The initial destination host for the datagram may be a router, gateway, or firewall between a LAN and the internet. Upon arrival at the device on the border between the LAN and the internet, the datagram may be opened, examined or decrypted in whole or in part, analyzed for further address information, and routed to a local IP address on the LAN.
The ISAKMP handshaking protocol used in IPSec requires that both hosts intending to establish a secure session between them use a process-specific port address (Port 500) for initial message exchanges. For this reason, Port 500 has been assigned for exclusive use with the ISAKMP protocol. By convention, computers attempting to negotiate secure communications parameters by employing the ISAKMP protocol must communicate strictly through each computer's Port 500. That is, ISAKMP messages from either computer must identify Port 500 as both the source and destination port addresses. If either computer receives a packet in which Port 500 is not specified as being both the source and destination, the packet will be discarded.
While this protocol provides assurance that two hosts are communicating with each other, it becomes unworkable when one host is located on a LAN that uses local IP addresses and a NAT gateway. For example, Host A, having a local IP address on a remote LAN protected by a NAT gateway, wishes to establish a secure session with Host B, located at a main office computing site. Host A would initiate the protocol by sending an unencrypted UDP datagram to Host B, giving the “destination” as Host B's IP address, and the destination port address as “Port 500.” However, when the datagram reaches the NAT gateway connecting the remote LAN to the internet, the gateway will translate the destination port address to an arbitrary port number. Upon the arrival of the datagram at Host B, the ISAKMP protocol will not be recognized, and Host B will not respond. The computers will fail to establish a secure session. Because of this difficulty, it has heretofore been believed that the ISAKMP protocol cannot be used to establish a VPN using a NAT gateway where each computer on the remote LAN uses a local rather than a global IP address.
It is therefore an object of this invention to provide a gateway that will permit the use of ISAKMP protocol authentication and key exchanges between a computer having a non-global IP address and a host computer, using the internet as a transmission medium.
It is a further object of this invention to provide a gateway that will allow any number of computers on a private LAN using local IP addresses to initiate or receive messages via the internet using ISAKMP protocol.
It is another object of this invention to provide a method for employing virtual private networking between two or more LAN sites on the internet, using ISAKMP protocol to initiate secure communications.
These and other objects of the invention will become apparent throughout the following description.